1. Field of the Invention
The present invention pertains to a novel system for monitoring various properties of flowing fluid streams and more particularly relates to an instrument and a method, using a multiplicity of electrical capacitors in a capacitance measuring bridge circuit, for the continuous inspection and monitoring of flowing fluid streams and comparing the dielectric properties of such streams with a reference fluid.
2. Description of the Prior Art
Many techniques have been suggested and developed to sample flowing fluid streams and compare stream samples with a reference fluid. In the prior art it is common to provide apparatus for measuring the conductivity of liquids, such as oils and the like, to determine the amount of impurities therein. If the impurities in the liquid have a different conductivity than that of the liquid, the conductivity of the combination of liquid and impurity will be altered with respect to the conductivity of the pure liquid and the difference in conductivities can be measured and reported. However, many impurities associated with or introduced into flowing liquid streams do not alter the conductivity of the liquid and thus may not be detected by flow monitoring instruments measuring conductivity changes.
It has been suggested in a number of prior art patents and patent applications to monitor and inspect the flow of fluids in product pipe lines, particularly petroleum products, by the passage of a sample stream of the fluid through a cell which is connected in a circuit energized by constant and relatively high frequency alternating potential under conditions wherein the fluid sample stream functions as a dielectric medium for the cell. The character of the fluid is then detected as a function of the dielectric constant of the sample stream as it passes through the cell by measuring the potential developed across the cell. It has been found that, in carrying out dielectric constant detection methods for monitoring and inspecting the character of flowing fluids, changes in temperature and pressure affect the dielectric constant of the sample stream flowing through the cell.
To overcome the adverse effects of changes of temperature and pressure on the dielectric constant of samples of flowing fluids, instruments have been designed to include dual capacitor cells which are connected in a capacitance bridge circuit including a sensing cell through which the sample stream is passed and a reference cell which contains a fluid having a known desired dielectric constant against which the dielectric constant of the sample stream is compared. By maintaining the two cells in intimate thermal contact with the sample stream, and by equalizing the pressure therebetween, changes in the dielectric constant of the sampled fluid due to temperature or pressure variations, which otherwise might promote spurious results and erroneously cause recording equipment to indicate a change of constitution of the sampled stream, can be compensated for to some extent. An instrument designed in accordance with the foregoing has been disclosed in U.S. Pat. No. 2,800,628 granted to L. W. Stinson et al on July 23, 1957.
Reference to the Stinson et al patent will reveal that the two capacitor cells of the instrument are each comprised of a single capacitor and that such capacitors are connected as the two variable capacitors in classic capacitive Wheatstone bridge circuitry. The remaining two capacitors of the Wheatstone bridge (fixed impedance value) are removed from the environment (fluid characteristics, pressure and temperature) of the two capacitor cells of the instrument. Thus, the entire bridge circuitry remains sensitive to temperature and pressure differences. Also, long lead lengths and moving leads allow stray capacitance pickup and overall variations in circuit impedence.